gpu: Use explicit atomics instead of assuming that 32bit read/writes are atomic.

gpu/command_buffer/service/query_manager.cc

Index: gpu/command_buffer/service/query_manager.cc
diff --git a/gpu/command_buffer/service/query_manager.cc b/gpu/command_buffer/service/query_manager.cc
index 85e273c6c3a02b570cc87b634823c577eaaf63a8..44fb8bb6ffeca49e6771a3f2236a5c4bd69fa037 100644
--- a/gpu/command_buffer/service/query_manager.cc
+++ b/gpu/command_buffer/service/query_manager.cc
@@ -24,7 +24,7 @@ namespace {
 class AsyncPixelTransferCompletionObserverImpl
     : public AsyncPixelTransferCompletionObserver {
  public:
-  AsyncPixelTransferCompletionObserverImpl(uint32 submit_count)
+  AsyncPixelTransferCompletionObserverImpl(int32 submit_count)
       : submit_count_(submit_count),
         cancelled_(false) {}
 
@@ -42,17 +42,14 @@ class AsyncPixelTransferCompletionObserverImpl
                    mem_params.shm_data_offset;
       QuerySync* sync = static_cast<QuerySync*>(data);
 
-      // Need a MemoryBarrier here to ensure that upload completed before
-      // submit_count was written to sync->process_count.
-      base::subtle::MemoryBarrier();
-      sync->process_count = submit_count_;
+      base::subtle::Release_Store(&sync->process_count, submit_count_);
     }
   }
 
  private:
   virtual ~AsyncPixelTransferCompletionObserverImpl() {}
 
-  uint32 submit_count_;
+  int32 submit_count_;
 
   base::Lock lock_;
   bool cancelled_;
@@ -68,7 +65,7 @@ class AsyncPixelTransfersCompletedQuery
       QueryManager* manager, GLenum target, int32 shm_id, uint32 shm_offset);
 
   virtual bool Begin() OVERRIDE;
-  virtual bool End(uint32 submit_count) OVERRIDE;
+  virtual bool End(int32 submit_count) OVERRIDE;
   virtual bool Process() OVERRIDE;
   virtual void Destroy(bool have_context) OVERRIDE;
 
@@ -87,7 +84,7 @@ bool AsyncPixelTransfersCompletedQuery::Begin() {
   return true;
 }
 
-bool AsyncPixelTransfersCompletedQuery::End(uint32 submit_count) {
+bool AsyncPixelTransfersCompletedQuery::End(int32 submit_count) {
   AsyncMemoryParams mem_params;
   // Get the real shared memory since it might need to be duped to prevent
   // use-after-free of the memory.
@@ -119,7 +116,7 @@ bool AsyncPixelTransfersCompletedQuery::Process() {
   // Check if completion callback has been run. sync->process_count atomicity
   // is guaranteed as this is already used to notify client of a completed
   // query.
-  if (sync->process_count != submit_count())
+  if (base::subtle::Acquire_Load(&sync->process_count) != submit_count())
     return true;
 
   UnmarkAsPending();
@@ -145,7 +142,7 @@ class AllSamplesPassedQuery : public QueryManager::Query {
       QueryManager* manager, GLenum target, int32 shm_id, uint32 shm_offset,
       GLuint service_id);
   virtual bool Begin() OVERRIDE;
-  virtual bool End(uint32 submit_count) OVERRIDE;
+  virtual bool End(int32 submit_count) OVERRIDE;
   virtual bool Process() OVERRIDE;
   virtual void Destroy(bool have_context) OVERRIDE;
 
@@ -169,7 +166,7 @@ bool AllSamplesPassedQuery::Begin() {
   return true;
 }
 
-bool AllSamplesPassedQuery::End(uint32 submit_count) {
+bool AllSamplesPassedQuery::End(int32 submit_count) {
   EndQueryHelper(target());
   return AddToPendingQueue(submit_count);
 }
@@ -204,7 +201,7 @@ class CommandsIssuedQuery : public QueryManager::Query {
       QueryManager* manager, GLenum target, int32 shm_id, uint32 shm_offset);
 
   virtual bool Begin() OVERRIDE;
-  virtual bool End(uint32 submit_count) OVERRIDE;
+  virtual bool End(int32 submit_count) OVERRIDE;
   virtual bool Process() OVERRIDE;
   virtual void Destroy(bool have_context) OVERRIDE;
 
@@ -225,7 +222,7 @@ bool CommandsIssuedQuery::Begin() {
   return true;
 }
 
-bool CommandsIssuedQuery::End(uint32 submit_count) {
+bool CommandsIssuedQuery::End(int32 submit_count) {
   base::TimeDelta elapsed = base::TimeTicks::HighResNow() - begin_time_;
   MarkAsPending(submit_count);
   return MarkAsCompleted(elapsed.InMicroseconds());
@@ -251,7 +248,7 @@ class CommandLatencyQuery : public QueryManager::Query {
       QueryManager* manager, GLenum target, int32 shm_id, uint32 shm_offset);
 
   virtual bool Begin() OVERRIDE;
-  virtual bool End(uint32 submit_count) OVERRIDE;
+  virtual bool End(int32 submit_count) OVERRIDE;
   virtual bool Process() OVERRIDE;
   virtual void Destroy(bool have_context) OVERRIDE;
 
@@ -268,7 +265,7 @@ bool CommandLatencyQuery::Begin() {
     return true;
 }
 
-bool CommandLatencyQuery::End(uint32 submit_count) {
+bool CommandLatencyQuery::End(int32 submit_count) {
     base::TimeDelta now = base::TimeTicks::HighResNow() - base::TimeTicks();
     MarkAsPending(submit_count);
     return MarkAsCompleted(now.InMicroseconds());
@@ -297,7 +294,7 @@ class AsyncReadPixelsCompletedQuery
       QueryManager* manager, GLenum target, int32 shm_id, uint32 shm_offset);
 
   virtual bool Begin() OVERRIDE;
-  virtual bool End(uint32 submit_count) OVERRIDE;
+  virtual bool End(int32 submit_count) OVERRIDE;
   virtual bool Process() OVERRIDE;
   virtual void Destroy(bool have_context) OVERRIDE;
 
@@ -315,7 +312,7 @@ bool AsyncReadPixelsCompletedQuery::Begin() {
   return true;
 }
 
-bool AsyncReadPixelsCompletedQuery::End(uint32 submit_count) {
+bool AsyncReadPixelsCompletedQuery::End(int32 submit_count) {
   if (!AddToPendingQueue(submit_count)) {
     return false;
   }
@@ -350,7 +347,7 @@ class GetErrorQuery : public QueryManager::Query {
       QueryManager* manager, GLenum target, int32 shm_id, uint32 shm_offset);
 
   virtual bool Begin() OVERRIDE;
-  virtual bool End(uint32 submit_count) OVERRIDE;
+  virtual bool End(int32 submit_count) OVERRIDE;
   virtual bool Process() OVERRIDE;
   virtual void Destroy(bool have_context) OVERRIDE;
 
@@ -369,7 +366,7 @@ bool GetErrorQuery::Begin() {
   return true;
 }
 
-bool GetErrorQuery::End(uint32 submit_count) {
+bool GetErrorQuery::End(int32 submit_count) {
   MarkAsPending(submit_count);
   return MarkAsCompleted(manager()->decoder()->GetErrorState()->GetGLError());
 }
@@ -563,10 +560,7 @@ bool QueryManager::Query::MarkAsCompleted(uint64 result) {
 
   pending_ = false;
   sync->result = result;
-  // Need a MemoryBarrier here so that sync->result is written before
-  // sync->process_count.
-  base::subtle::MemoryBarrier();
-  sync->process_count = submit_count_;
+  base::subtle::Release_Store(&sync->process_count, submit_count_);
 
   return true;
 }
@@ -611,7 +605,7 @@ bool QueryManager::HavePendingTransferQueries() {
   return !pending_transfer_queries_.empty();
 }
 
-bool QueryManager::AddPendingQuery(Query* query, uint32 submit_count) {
+bool QueryManager::AddPendingQuery(Query* query, int32 submit_count) {
   DCHECK(query);
   DCHECK(!query->IsDeleted());
   if (!RemovePendingQuery(query)) {
@@ -622,7 +616,7 @@ bool QueryManager::AddPendingQuery(Query* query, uint32 submit_count) {
   return true;
 }
 
-bool QueryManager::AddPendingTransferQuery(Query* query, uint32 submit_count) {
+bool QueryManager::AddPendingTransferQuery(Query* query, int32 submit_count) {
   DCHECK(query);
   DCHECK(!query->IsDeleted());
   if (!RemovePendingQuery(query)) {
@@ -668,7 +662,7 @@ bool QueryManager::BeginQuery(Query* query) {
   return query->Begin();
 }
 
-bool QueryManager::EndQuery(Query* query, uint32 submit_count) {
+bool QueryManager::EndQuery(Query* query, int32 submit_count) {
   DCHECK(query);
   if (!RemovePendingQuery(query)) {
     return false;